The use of orbiting satellites is an integral part of today's worldwide communications systems. As the technology and hardware of such systems continues to advance significantly, it is expected that satellites will continue to play an ever-increasing role in the future of long-range communications. Each new generation of satellites has been more technologically sophisticated than its predecessors, and each has had a significant impact on the development and capabilities of military, domestic and international communications systems. This progress is expected to continue as new developments in satellite communications systems occur in the future.
Today's satellite systems can perform a wide variety of functions, besides the basic operation of completing a long-range communications link. For example, satellite systems may be used for navigation and position location, weather monitoring, terrain observations, and deep-space exploration, and are an integral part of wide area distribution networks. Other, even more sophisticated uses for satellite systems are being investigated.
A satellite communications system may take on several different forms. Typically, such systems comprise an uplink from a ground-based earth station to a satellite, and a downlink from the satellite back to another earth station. Ground-based earth stations may be designated as a transmitting station only, or a receiving station only, but more commonly these ground-based earth stations are designated as transmitting-receiving stations.
The internal electronics of an earth station are conceptually quite simple. In a transmitting portion of a source station, baseband information from a user information source, such as a telephone, television, facsimile or computer, are brought in on cable or microwave link from the various sources. The baseband information is then multiplexed (combined) and modulated by a modulator onto a sinusoidal intermediate frequency (IF) carrier signal.
The IF carrier signal is typically a bandpass signal which facilitates data transmission to a much greater extent than a baseband signal and is therefore the preferred signal format in long range data satellite communications systems. The modulator of the source station functionally operates to shape the baseband data signal and combine the resulting shaped signal with the sinusoidal IF carrier signal to provide a data bearing information signal operating at the carrier frequency. Shaping is performed to provide the data bearing signal appropriate spectral properties which facilitate transmission.
The data bearing IF carrier signal is then translated to radio frequencies (RF) for power amplification and transmission through the atmosphere to the satellite. The satellite receives the RF signal from the source earth station and amplifies and conditions the signal. The satellite then relays the amplified and conditioned signal to a receiving portion of a receiving earth station.
At the receiving station, the RF signal is first translated back to IF. At the IF, uplink data bearing carrier signal is typically further filtered and then demodulated by a demodulator to recover the baseband source waveforms. The demodulator at the receiving station reverses the process performed by the modulator at the source station by recovering the originally transmitted baseband signal from the carrier frequency.
The demodulator located within the receiving portion of the receiving station typically includes a downconverter which is a device that converts the higher frequency of the RF-to-IF translated signal to a lower frequency by mixing it with a local baseband frequency. The mixing process, known as heterodyning, produces frequencies corresponding to the sum and the difference of the two original frequencies. The output of the downconverter is the difference, or lower frequency, signal. The downconverter thus converts a bandpass signal to a baseband signal for further processing by the demodulator.
FIG. 1 shows the implementation of a known downconverter for a satellite communications system. The construction and operation of the known downconverter is as follows. The received analog IF bandpass signal is received by a series of bandpass filters 1 which filter the signal to eliminate unwanted signal variations which may have been introduced during transmission of the RF signal from the satellite through the atmosphere to the receiving station, or during translation of the RF signal to the IF signal. The bandpass filters may be constructed as shown in U.S. Pat. No. 5,191,305 to Frost, et al., incorporated by reference herein. An automatic gain control circuit 2 is employed to provide a consistent signal to an analog mixer configuration comprising a carrier frequency source 3, a phase shifter 4 and a pair of mixers 5a-5b.
The output of the mixers is a baseband signal comprising a real or inphase part (I) and an imaginary or quadrature part (Q). The baseband signal is recovered from low pass filters 6a and 6b, respectively, and digitized by analog-to-digital (A/D) converters 7a and 7b, respectively. The digitized baseband signal may then be further processed by the demodulator, for example by further filtering the digitized signal.
Known downconverter circuits such as the one shown in FIG. 1 are typically implemented in analog hardware. For example, see U.S. Pat. No. 5,179,730 to Loper. In such known downconverter circuits, the processes of signal mixing, signal filtering, and automatic gain control are all performed prior to converting the signal into a digital format. The received bandpass signal is converted to digital format only after being initially filtered, mixed with a baseband frequency to separate the signal into its inphase and quadrature components, and filtered to recover the baseband signal.
Such extensive use of analog circuitry in constructing a direct downconverter inherently results in device deficiencies. For example, instability associated with analog signal drift is common with such circuits. In addition, power required by analog circuitry is typically greater than the power associated with digital circuitry.
Accordingly, it is an object of the present invention to provide a reliable and easily maintainable digital downconverter circuit for use in a satellite communications demodulator. It is a further object to provide such a digital downconverter which simplifies the circuitry, increases stability by minimizing signal drift, increases known speeds of operation, and reduces the power requirements of corresponding analog devices.